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100576.1776@compuserve.com (Antony Pace (UK/CH)) writes:

>Hi all,

>I am having problems writing a macro (Ansi C) for which I feel there
>must be standard technique.  Can anyone help?

[some text edited]

>#define CLEAR_ERROR(e) { \
>  (e).a = 0;             \
>  (e).b = 0;             \
>  (e).c = 0;             \
>  TRACE("clear error");  \
>}

[example of a situation where "CLEAR_ERROR(foo);" works edited]

>Note how the semicolon I put after the invocation becomes a null
>statement.  I expect users of my CLEAR_ERROR macro to use it, as then
>the macro looks like a function call.

>In this code segment, my macro does not work:

>  ...
>  if (a==3)
>    CLEAR_ERROR(error);
>  else
>    b=5;
>  ...

This is why "function like" macros should not be defined using "raw"
braces. A useful solution is to put the "body" of the macro in a
"do { ... } while(0)" loop.
 
#define CLEAR_ERROR(e) do { \
  (e).a = 0;             \
  (e).b = 0;             \
  (e).c = 0;             \
  TRACE("clear error");  \
} while (0)

Now you _need_ a semicolon after the "invocation" of the macro. I would
not dare to claim that this is a "standard technique", but the only
potential problem I see is that you have to persuade lint that you
_really_ want a constant conditional expression in that while statement.

Kurt
-- 
| Kurt Watzka                             Phone : +49-89-2180-6254
| watzka@stat.uni-muenchen.de
| ua302aa@sunmail.lrz-muenchen.de